Signaling system



April 30, 1940. V H P, sw u 2,198,901

SIGNALING SYSTEM Filed March 17, 1958 5 Sheets-Sheet 2 FIG. Z. 2205 r- ZZ 2 o 207 I 2 2 204 235- I l it 206 k 2/6 BY SW ism A TTORNEYS April 30, 1940. H. P. BOSWAU 2,198,901

SIGNALING SYSTEM Filed March 17, 1938 I5 Sheets-Sheet 3 [N VEN TOR b'fllVS P. 306M990.

A TTORNE Y5 Patented Apr. 30, 1940 SIGNALING SYSTEM Hans P. Boswau, Lorain, Ohio, assignor to The Lorain Telephone Company, Lorain, Ohio, a

corporation of Ohio Application March 17, 1938, Serial No..196,467

7 Claims.

This invention relates to signaling systems, and in particular, to selective signaling systems for selective communication between one or more master stations and a large number of subsidiary stations.

One object of this invention is to provide a selective signaling system for signaling between one or more master stations and a large number of subsidiary stations, such as ship radio stations or way stations on long telephone lines in a rapid, simple and dependable selective ar-= rangement. I

Another object is to provide a selective. telephonic signaling system, giving substantially instantaneous selection and response of the calling device at the called station, thereby eliminating thedelay resulting in prior systems from the employment of long and extended trains of selecting impulses, and at the same time enabling the employment of code ringing signals for transmitting additional information with the calling signal.

'Another object is to provide a selective telephonic signaling system, wherein receiving stations are enabled to use standard telephone devices without the necessity for the provision of special selector mechanisms of a costly, intricate and delicate nature.

Another object is to provide a selective telephonic signaling system giving means for making simple and inexpensive circuits. for facilitating calls from one subsidiary station to another subsidiary station.

In the drawings:

circuit at the master station.

Figure 2 is a wiring diagram of the receiving equipment at ship stations, or other subsidiary stations.

Figure 3 is a modified subcircuit" applicable to Figure 2, showing a modified filter circuit adapted for ship call and fleet call, or ship call and general call purposes.

Figure 4 is a modified. subcircuit applicable to Figure 2, showing another modified filter circuit adapted for ship, fieet and general call purposes. Figure 5 is a wiring diagram of the sending equipment at the ship stations, or other subsidiary stations.

Figure 1 is a wiring diagram of the sending General nature In general, the radio signaling system of the present invention consists of a master circuit and a plurality of subsidiary circuits arranged for intercommunication. For calling purposes in this signaling system, the carrier wave of the radio transmitter is modulated by a selective ringing signal consisting of alternate impulses of two different audio frequencies, alternating 10 at suitable harmonic or synchromonic ringing frequencies. According to this arrangement one of the audio frequencies is transmitted during one-half of the period of the ringing frequency correspondingfor example, to the positive half 5 cycle and the other audio frequency is transmitted during the second half period corresponding, for example, to the negative half cycle.

The system'is arranged so that any one of a large number of subsidiary stations may be called 20 selectively. In particular, assuming the possibility of twenty audio frequencies for transmission upon one voice channel and segregated from each other by suitable filters at the receiving station, a total of 190 different combinations of two 25 of these twenty frequencies may be obtained, as found from the simple computation: I

Furthermore, as each of these 190 combinations 30 may be alternated at one of the five customary harmonic or synchromonic ringing frequencies, the product of these numbers gives a total of 950 different calling signals available for the selective calling of any one of 950 stations. 35

In setting forth the invention, the equipment at each station will first be described, and then the operation of the system as a whole. The reference numerals in each figure are numbered I in hundreds according to the particular figure. 40 That is, all reference numerals in Figure 1 are numbered between and 199, Figure 2 between 200 andv 299, etc.

Master station arrangement The master station is provided with sending equipment, as shown in the wiring diagram of Figure 1. This sending equipment includes flve keys IIII, I02, I03, Inland I05 for selecting the inclusive, and I30 to I38, inclusive. These audio frequency generators may be of any suitable type for providing such oscillations such as, for example, a rotating machine used for voice frequency telegraph systems or vacuum tube oscillators.

In order to provide the ringing frequencies employed in the system, five ringing frequency generators I to I45, inclusive, are connected in circuit with the ringing frequency selecting keys IN to I05, inclusive, These ringing frequency generators are shown for purposes of illustration to consist of mechanically tuned vibrators operated electrically, in the manner in which such devices are employed in wired telephone exchanses.- It will be obvious to those skilled in the art, however, that five motor-driven commutators could be employed in place of the five ringing frequency generators I to I45.

Also provided in the circuit of ,Figure 1 is a starting key I45 and two starting relays I41 and I48, these being employed for the purpose of starting the ringing frequency generators I to I45 in vibration. A transformer I49 inductively couples the signal and ringing frequency circuits to the radio transmitter, generally designated I50, the latter being-connected to an antenna I5I and ground I52. Arranged between the transformer I48 and radio transmitter I50 is a ringing key I53, the purpose of which is to disconnect the radio transmitter I50 from the telephone circuit I54 and to connect it to the trans former I48. Additionally provided in the circuit of Figure 1 is a low audio frequency generator I55, together with a tone key I58 connected thereto for the purpose subsequently to bedescribed.

Ship receiving arrangement The receiving circuit at the ship or subsidiary station (Figure 2) consists of a radio receiver 20 I connected to an antenna 202 and a ground 203. The switch hook 204 serves to switch the output of the radio receiver 20I from the signaling apparatus .to the telephone circuit 205, the latter being connected to a signal indicator, such as a telephone receiver or loud speaker.

The signal-selecting apparatus in the subsidiary station receiving equipment includes an input transformer 206 connected to two audio frequency filters, generally designated 201 and 208, by way of the conductors 235 and 238. The filters 201 and 208 include inductances 208 and 2, coupled to capacities 2I0 and 2I2, respectively. The audio voltage arising at the filters 201 and 208 is transmitted to other lines 231,

238 and 238, and rectified by the rectifiers, generally designated 2I3 and 2. Beyond these it is fed to the windings 2I6 and 2I1 of a polarized relay, generally designated 2 I5, the latter having a holding winding 2| 8 and an armature 2I8 with associated contacts. A ringer 220 of the usual harmonic or synchromonic type is operated from the power supply of the station over the lines 225 and 221, the circuit including resistances 222 and 223, a condenser 22I and resistances 224 and 5 cooperating with the holding winding I8 of the polarized relay 2I5.' Optionally provided in the receiving circuit of Figure 2 is an additional signaling device connected between the radio receiver 20I and the transformer 208. This includes the transformers 228 and 234, the filter inductances 228 and 230 and condensers 23I, 232 and 233. A signaling device 242. such as a loud speaker, may be optionally provided, as set forth more fully below. Optionally connected beyond the first ringer 220 and in parallel therewith is a second ringer 2, for purposes set forth more fully below.

The circuit shown in Figure 2 includes a filter system composed of filters 281 and 208, adapted to handle the ship call only. Figure 3, however, shows a filter system adapted to handle either a ship call and a fleet call, or a ship call and a general call. This filter system is adapted to be substituted for the filter system inclosed in the dashed lines in Figure 2, andfor that purpose is connected between the Junction of the lines 235 and 231 and the Junction of the lines 238 and 238. The line 238, as before, leads from the midportion of the filter system. The filter system of Figure 3 includes two pairs of filters "I and 303, and 302 and 304. The filters, as before, include inductances 308, 3i I, 3I3 and 3I5 in circuit with capacities 3I0, 3I2, 3I.4 and 3I8.

Figure 4 shows a filter system arranged for substitution in the dashed inclosure of Figure 2. and adapted for handling ship, fieet and general calls. This filter system of Figure 4 includes three pairs of 'filters 40I and 404, 402 and 405, and 403- and 408, connected in a similar manner to those of Figure 3 between the junction of the lines 235 and 231 and the junction of the lines 236 and 238. These filters 40I to 408, inclusive,

Ship sending arrangement The ship sendingcircuii'.v for use in the radio signaling system of this invention is shown in Figure 5. This circuit includes an audio frequency oscillator consisting of a vacuum tube 50I connected to a transformer 502, the latter being further connected to tuning condensers 503 and 504. Also included in the circuit of Figure 5 is a choke 506, a by-pass condenser 501, a cathode bias resistance 508 and a by-Dass condenser 508. A ringing frequency vibrator 5I0 is connected in this circuit and provided with an interrupting contact 5 and an associated spark quenching condenser 5I2, likewise an operating contact 5I3.

The ringing frequency vibrator 5I0 is started by means of the switch hook 5, whereas the audio frequency oscillator is started by closing a ringing key 5I5, thereby closing the plate voltage circuit of the audio frequency oscillator and switching the input circuit of the radio transmitter, generally designated 5I1, from the microphone circuit 5I8 to the output of the audio frequency oscillator. The radio transmitter 5I1 is connected to an antenna 5I8 and a ground 5I8. The filament or heater of the vacuum tube 50I is energized from a heater supply circuit 520. A power supply circuit 52 I, of any suitable voltage such as volts, is connected to the ringing frequency vibrator 5I0 in the manner shown in Figure 5.

The ringing vibrators I to I45 may be arranged to vibrate at any desired series of frequencies. Similarly, the audio frequency generators I40 to I 40, inclusive, may be arranged 75 for purposes of exempliflcation, the ringing frequency generators I to I45 have been indicated -as having frequencies of 16, 30, 42, 54 and 66 cycles per second, respectively. Similarly, the audio frequency generators I40 have been designated with audio frequencies extending from 420 to 2700 cycles per second. Placing definite values for these respective frequencies enables a clearer showing to be made, but it is understood that the invention is not limited to the particular numbers of cycles described.

Each of the ringing frequency generators I4I to I45 is provided with a vibrator IIiI to IGI respectively, and also with operating coils l51 to I 51, respectively. Each ringing frequency generator I to I 45 is further provided with a normallyclosed contactJIiw to I 60, inclusive. likewise a condenser I50 to I50, inclusive, and a resistance I58 to I 58, inclusive. For purposes of illustration, the signal tone generator I55 is designated as having a frequency of 300 cycles per second.

Operation If the operator at the master station wishes to call a particular ship of the system, he first operates the starting key I46 (Figure 1) to start the ringing frequency generators I to When the key I46 is closed it operates the relay I40 through a break contact I41 of the relay I41. When the relay I48 is thus energized, it closes individual circuits to operating coils I51 to I51 of the ringing frequency generators I to I45, and these, when energized, attract their reed armatures- IGI to I5I'=, inclusive.

When the relay I48 closes it likewise operates the relay I41, which locks itself to a contact of the key I46 and opens the circuit of the relay I40. The latter, however, is a slow release relay and remains operated for a predetermined time after it has become deenergized. During this time lag interval the coils I51 to I51 attract the armatures IIiI to I6I'. When the relay I 48 releases its contacts after this predetermined time, the coils I51 to I51 thenceforth receive current through the contacts I50 to I60,

resistances I58 to I58 and the make contact I41 of the relay I41, causing the reed armatures I6I= to IOI to vibrate in the manner of an ordinary electric bell. The reed armatures I6I are tuned mechanically sothat they vibrate at predetermined desired frequencies, such as the frequencies 16, 30, 42, 54 and 66 cycles per second shown in Figure 1. The starting relay I48 is provided to produce a strong initial pull upon the reed armatures IBI" to I6 I' by means of the coils I51 to I51 thereby insuring starting at full amplitude of vibration. Condensers I59 to I50 are provided for spark protection to the contacts I60 to. "50, inclusive. The left-hand contact I41 is a make contact which may be used to energize the circuit starting audio frequency generators I40 to I40, inclusive.

In the signaling system of the present invention each ship is assigned a call number consisting of four digits. The first digit indicates one of the keys IM to I05, the second digit one of the keys III to II3, the third digit one of the keys I20 to I29, and the fourth digit one of the keys I30 to I 39. The numerals of the call number for the particularship, therefore, correspond to the last digits of the key numbers. For exkeys I04, m, m and m. If desired, each of the four groups of keys mentioned above may be equipped with a mechanical interlocking arrangement so that the operation of any one key will restore any previqusly operated key in the same group to its inoperative position.

The equipment is now ready for signaling, and the operator now signals the desired ship by means of the ringing key I53 (Figure 1). The latter connects the radio transmitter I50 through the transformer I49 to the vibrating reeds IGI' to I5I', inclusive, of all five ringing frequency generators I H to I45, inclusive. As the contact circuits associated with the reeds IBI', IBI", I6I and I6I are opened at the contacts of the keys "I, I02, I03 and I05 only the contact circuits of the ringing frequency generator I44 are closed through the operated key I04.

When thus energized, the vibrating reed armature IIiI connects the transformer I43 alternately to the left andright-hand contacts of the generator at a frequency, for example. of 54 cycles per second. When the right-hand contact of the ringing frequency'generator I44 is closed the transformer I49 is connected through the contacts of the keys I I2 and I23 to the audio frequency generator I40 for an audio frequency of 780'cycles. On the other hand, when the left-hand contact thereof is. closed, the circuit is completed through the contacts of the keys I I2. and I31 to the audio frequency generator I40 for an audio frequency of 2460 cycles.

As a result'of these connections, signal impulses consisting" of fifty-four impulses persecond of 780 cycles, interspersed by fifty-four impulses per second of 2460 cycles are impressed upon the radio transmitter I50, which radiates a. signal modulated in a corresponding manner. By using any one of the ringing frequencies and any two of the audio frequencies in a different combination, any other combinationof operated selection keys will transmit a corresponding signal. For example, when the key III is operated the right and left-hand contacts of the ringing frequency generators I4I to I45 are both associated with the ten audio frequencies ranging from 420 to 1500 cycles per second. Since any one of these ten frequencies can be combined with any one of the nine remaining frequencies, a total number of combinations of is achieved. Since, however, the frequency combinations composed of the same number of cycles in reverse arrangement cannot be distinguished from one another, only half of the 90 combinations or 45 combinations are distinct from each other. Thus, for-example, the frequency combination of 420 cycles with 540 cycles would be indistinguishable from the combination of 540 cycles with 420 cycles.

If, however, the second key ;II2 is operated instead of the first key III, the right-hand 'contact of the vibrators I to I45'is connected to become associated with the ten au'dio frequencies ranging from 420 to 1500 cycles per second, and the left-hand contact with the ten audio frequencies of 1620 to 2700 cycles per second, thereby giving one hundred distinct combinations. If the third key H3 is operated it associates both vibrator contacts with the ten audio frequencies of 1620 to 2700 cycles per second so that similar total number of audio frequency combinationsv for a given ringing frequency, therefore, amounts to 190. The call numbers corresponding to these different audio frequency combinations obtained by operating the first ringing frequency key l0i are given in the following table:

By using the other four ringing frequencies a similar table will be obtained, having the numbers 2, 3, 4 or 5 for the first digit of the call number instead of the No. 1. These other ringing frequencies add 760 distinctive signals to those ob- 4 tained by the first ringing frequency, giving a,

is connected, through the switch hook 204 and transformer 206, to the two audio filters 201 and 208 in series. Onany particular ship one of these filters is tuned to one of the two audio frequencies us@for calling this ship, and the other filter is tuned to the other audio frequency. The output of these filters passes through the condensers 240 to the full wave rectifiers H3 and 2. In these rectlfiers audio frequencies are rectified and the resulting rectified current is fed to the windings 2 I6 and 2H of the polarized relay H5. The connections between the rectifiers 2i3 and 2M and the relay windings 2|6 and 2H are made in such a manner that an audio frequency signal of the frequency to which the filter 201 is tuned operates the relay M5 in one direction, whereas a signal of the frequency to which the filter 208 is tuned operates the relay 2 I 5 in the opposite direction. The additional winding 2i8 of the relay M5 in cooperation with the resistances 224 and 225, provides a direct current bias which reverses with the reversal of the relay armature 2i9, thereby tending to hold the armature 2 I 9 against the contact-to which it was last moved.

Assuming that the ringing frequency selected is that of the ringing frequency generator I with a frequency of 54 cycles per second, the alternating impulses in the windings 2H3 and 2 ll of the relay 2i! move the armature 2i! thereof back and forth fifty-four times per second. This action alternately charges and discharges the condenser 22i through the winding of the harmonic ringer 220 and resistances 222 and 223. The effect of this action is to produce an alternating current of fifty-four cycles flowing through the ringer 220. The latter, however, is mechanically tuned to respond only to impulses of alternate direction at a predetermined single frequency. Thus the ship being called, in the particular example, has its ringer 220 mechanically tuned to respond to 54 cycles per second when the ringer will ring in response to the arrival of the call through the ship's receiving set, shown in Figure 2. a

It will be evident-that the same combination of the two identical audio frequencies may be used for calling four other ships of the same system because their ringers 220 are not tuned to 54 cycles, and therefore do not respond to the signal even though it is received and rectified and aperates the relay 2". One or the other of the two audio frequencies will be used for calling a number of other ships, thereby causing the armature 2l9 of the relay 2; to move on the first impulse of the signal unless it is already engaged with the corresponding contact. The other audio frequency on these ships, however, will differ, that is, their second filter is not tuned to the other I through them in approximately equal amountsand is therefore substantially balanced out in the relay 2l5, since the rectified currents from the two rectifiers 2|! and 2H produce the magnetic fiux of opposing directions in the relay.

In this manner the relay M5 is not aflected by the impulses of the second audio frequency, with the result that the armature 2i! remains against the contact to which it was first moved. A single impulse will not operate the ringers 220 on these ships even when they receive such a single impulse because several cycles are required to'swing the clapper of the ringer a sufilcient amount to hit and sound the gongs. In all other ships, Whose filters M3 and 2 are tuned to neither of the two audio frequencies above mentioned, the relay 2 i5 will not be affected at all.

Noise or static interference will also leave the relay 2H5 substantially unaffected because it will pass through both filters in approximately equal amounts and balance out in the relay 2l5. If, however, occasional interference or static should contain one of the audio frequencies, the ringers 220 will not be affected, because their mechanical tuning requires the reception of several impulses at the proper ringing frequency before they begin to ring. Similarly, the signal on the ship being called will not be affected materially by static or interference asthis can have no worse effect than to obliterate the signal during a few cycles of the ringing frequency. As the mechanical inertia of the vibrating system of the ringer easily carries it over such brief interruptions, no effect will ordinarily be detected. Even if the noise or static interferences are long enough to break the swing of the ringer 220, the result will simply be a somewhat uneven ring.

In the prior so-called selector system of selective calling, however, one missing impulse will spoil the call. The superiority of the present system over the prior system is therefore selfevident. Therefore, in summation, it will be seen that in order to call a particular ship of the system, it is necessary not only to transmit to it the two audio frequencies to which its two audio filters are tuned, but also to alternate these two frequencies at the frequency to which the ringer of the ship is tuned. The system of the present invention is adapted to call a large number of ships in a very small period of time, because the ringer 220 of each called ship responds almost instantly to the calling signal, and does not require the time-consuming operation of step-bystep relays, etc.

To provide a common calling signal for an entire fleet of ships, the filter arrangement of Figure 3 may be employed in the circuit of Figure 2. In this modified circuit the ringing frequency is the same for all ships of the fieet, but a second set of audio filters 302 and 304 is provided. These two additional filters are tuned to the same two audio frequencies on all ships of the particular fleet, thereby making it possible to callthe entire fleet at the same time. As neither of the two audio frequencies of such a fleet call can be used for other calls on the same ringing frequency, the number of calling code signals is reduced by l9+18=37 for each fleet called. If this were not so the filters 3M and 304 or 302 and 303 might respond to other signals.

As a further modification, by omitting the filter 304, one common audio frequency may be used for fleet and individual calls in a certain fleet. This arrangement provides for one fleet call and eighteen individual calls with the same ringing frequency of the ringers 220, but with different audio frequencies. If the fleet has more than eighteen ships, the other ships of the fleet may be called by using the same audio frequencies, and one or more additional ringing frequencies, that is with ringers 220 tuned to different ringing frequencies. This arrangement allows large fleets of ships to be called in groups of eighteen ships merely by employing a few successive ringing signals.

To make a general call to all ships in the system, two separate audio frequencies not used for any other calls may be reserved for this purpose. All ships in the system may then be signaled by employing these two reserved audio frequencies in combination with all five of the ringing frequencies in succession. In this manner. all of the ships in the system may be called in approximately five seconds. Similarly fleet calls may be provided by using individual code ringing signals for each fleet, and a simple code ringing signal, such as three long rings, for the general call. With the modification shown in Figure 4, fleet calls can be combined with a general call by providing three pairs of audio frequency filters. If, however, the fleet calls use an audio frequency in common with the individual calls of that fleet, one of the filters, such as the filter 405, may be omitted in a manner analogous to that previously described in connection with Figure 3. The filter circuit in Figure 4 is, of course, to be substituted for the filters 201 and 208 in Figure 2.

The use of the polarized relay 215 is for the purpose of permitting the ringer 220 to respond solely to the proper ringing frequency. If the ringer 220 were provided with two windings connected directly to the output of the rectifiers 213 and 2 I4, the ringer 220 would respond not only to its assigned signal, but also to any signal containing one of its two audio frequencies at the proper ringing frequency. The reason for this behavior arises from the fact that the ringer 220 responds not only to impulses of alternate polarity, but also to pulsating current of one polarity at the proper frequency. This is caused by the inertia and elasticity of the mechanically tuned,

armature of the ringer 220, which tend to restore this armature to normal after each impulse. The polarized relay 2|5, however, because of its bias winding 2 l9, remains in the last position to which it is moved until an opposite impulse is received. The possibility exists of providing the ringer with three windings and relay contacts, but the provision of a separate relay and ringer also requires much less energy for their operation, so that they can be operated directly from the output circuit of the radio receiver. A combined ringer and relay, however, would require addi' tional amplification, hence, the advantage of simplification in one respect would be offset by the greater complexity of the amplification system required. v

For ship-to-ship calling the master station calling equipment described above would be unnecessarily elaborate and expensive, except for sisting of the inductances 229 and 230 and the condensers "I, 232 and 233. The energy passing through this system is transformed by the transformer 234 .and passes to a signal indicator, such as the loud speaker 242.

The tone frequency used for ship-to-ship calling in this arrangement is lower than any of the selective audio frequencies, and hence, will not affect the selective calling apparatus. The selective audio frequencies are likewise kept from the, speaker 242 by the low pass filter system 229 to 233. In order to improve the signal reception, the speaker 242 is arranged to have its peak at the signal tone frequency.' 'If, however, there is provided a speaker or fhowler 242, which is mechanically tuned to the signal tone frequency, the low pass filter system 229 to 233 may be omitted, if desired. Optionally, in place of this simple low pass filter system, a band pass filter or a more elaborate low pass filter may be employed. Similarly, the audio frequency filter systems 201, 208, etc. may be replaced by more complex or improved forms of filters, such as diflerential band pass filters. Signal tone calling may be enabled by providing the master stations with a signal tone generator, such as the signal tone generator I55 and its associated key I56 (Figure 1). This may be employed when the ships are equipped for signal'tone calling in addition to selective calling.

- For ship-to-ship calls one common pair of audio frequencies and one ringing frequency only may be used. In this arrangement only two audio frequency generators and one ringing fre quency generator or one audio frequency generator furnishing two alternate audio frequencies under the control of a ringing vibrator would be required on the ships, as shownin Figure 5. In order to avoid complications, the ringing frequency in such an arrangement should not be used for selective calling from master stations, thereby making it necessary to connect a second harmonic ringer 2 in series or parallel with the ringer 220 on each ship. The ships then call each other by means of code ringing on the separate ringer 2. In an obvious manner, the shipto-ship ringing signal may likewise be used for fleet and general calls from master stations by using appropriate code ringing signals.

In the operation of the transmitting equipment for ships, as shown in Figure 5, when the telephone is lifted from the switch hook 5" the latter energizes the ringing frequency generator 510 by way of its interrupter contact 5| I, causing the armature 5i3to vibrate at the desired ringing frequency. Instead of using the switch hook 5, the ringing frequency generator 5|!) may be started by other suitable means, such as a separate switch. In order to call another ship the ringing key SIS is operated in accordance with the code ringing signal assigned to the ship being .called. The closing of the key 5|! starts the audio frequency oscillator of Figure 5 by closing its plate supply circuit. The audio frequency generated by this system is determined mainly by the inductance of the plate'winding B2! of the transformer 502 and the capacity 503 in parallel therewith.

When the ringing frequency generator vibrator H3 is in open circuit, as shown in Figure 5, the condenser Wlis arranged in parallel with the plate winding Ml, thereby generating a certain audio frequency. At intervals corresponding to the ringing frequency developed by the period of vibration of the armature M3, the lat-.

ter closes its contacts, thereby connecting the condenser 504 in parallel with the condenser 503 and lowering the audio frequency generated by the system. The output of the audio oscillator of Figure 5, therefore, provides for two audio frequencies alternating with each other at the ringing frequency of the armature H3. The audio output is connected to the radio transmitter 5i! whenever the key M5 is operated,,thus modulating the carrier wave of this transmitter with the desired calling signal at intervals controlled by the operation of the key M5. The latter may be replaced, if desired, by the dial controlled code sending means shown in my copending application, Ser. No. 130,943, filed March 15, 1937, as previously referred to. Similarly, any other type of audio frequency or ringing frequency generators may be employed in place of those shown, or two separate audio frequency generators may be used instead of the single audio frequency generator 5l0. The customary means for stabilizing the audio frequencies may likewise be added, such as voltage stabilizers, temperature compensators, etc.

,Additional information may be transmitted by this code ringing system in connection with the calling signals, because the ringers 220 and 2 respond to the calling signal almost instantly. For example, the channel on which a master station is calling may be identified by the use of one, two or more rings in the calling signal, and other codes may be employed in an obvious mannor to distinguish urgent calls from ordinary calls, or for any other purpose. In this manner the calling system of the present invention is much more flexible in meeting the varying signaling requirements than the previously known .-so-called selector system.

The harmonic ringers 220 and 2 at the receiving stations may be replaced by any other type of device which responds only to the predetermined ringing frequencies. In place of the electrical audio filters shown at the receiving station in Figure 2, any other suitable means respending only to predetermined audio frequencies may be employed, such as acoustic filters, mechanically tuned reed relays, etc.

Selective calling system ,for land lines circuit at some point between the telephone line and the relay 2 I I. The present system, moreover, is practically instantaneous, requires no delicate selector mechanism at the way stations and permits the use of code ringing signals to transmit additional information. Accordingly, it is decidedly superior to the previously known type of selective calling by means of train dispatching selectors. It will be understood that I desire to comprehend within my invention such modifications as come within the scope of the claims and the invention.

Having thus fully described my invention, what I claim as new and desire to secure by Letters Patent, is:

1. In a signaling system, a sending station, a plurality of receiving stations, means at said sending station for producing a plurality of paired sets ofelectrical oscillations of different frequencies, means at said sending station for transmitting said different frequency oscillation sets to said receiving stations, means at said sending station for alternating the transmission of said sets of oscillations at a plurality of predetermined rates of alternation, a signal indicator at each of said receiving stations, and means associated with said signal indicator for rendering one of said signal indicators operative solely in response to the arrival at said receiving stations of two sets of electrical oscillations of the same frequencies as those transmitted from the sending station and alternating at the same rate of transmission alternation.

2. In a signaling system. a sending station, a plurality of receiving stations, means at said sending station for producing a plurality of paired sets of electrical oscillations of different frequencies, means at said sending station for transmitting said different frequencies to said receiving stations, means at said sending station for alternating the transmission of said plurality of paired sets of oscillations-at a plurality of predetermined rates of alternation, a signal indicator at each of said receiving stations, said indicators being adapted to respond to alternations of said paired sets of oscillations, and a filter system at said receiving station responsive-solely to the same frequencies as said frequencies transmitted by said sending station for causing said signal indicator to respond to the alternations of said paired sets of oscillations.

3. In a signaling system, a sending station, a plurality of receiving stations, means at said sending station for producing a plurality of paired sets of electrical oscillations of different frequencies, means at said sending station for transmitting said different frequencies to said receiving stations, means at said sending station for alternating the transmission of said plurality of paired sets of oscillations at a plurality of predetermined rates of alternation, a signal indicator at each of. said receiving stations, said indicators being adapted to respond to the alternations of said paired sets of oscillations, a filter system at said receiving station capable solely of passing the same frequencies as said frequencies transmitted by said sending station, and means responsive solely to the transmitted rate of altion for transmitting said different frequencies to said receiving station, means at said sending station for alternating the transmission of said two sets of oscillations at a predetermined rate of alternation, a signal indicator including a polarized relay at said receiving station, and a filter system associated with said signal indicator and polarized relay for rendering said signal indicator operative solely in response to the arrival at said receiving station of two sets of electrical oscillations of the same frequencies as the two sets of electrical oscillations transmitted from said sending station, said polarized relay being adapted to remain in the last position to which it has been moved, said filter system being connected to said relay so that one set of alterna- 'tions will move the relay to one of its positions and the other set to its other position and said indicator being of a character as to respond only when the polarized relay is moved back and forth at a predetermined rate of alternation of the two sets of oscillations. v

5. In a signaling system, a sending station, a receiving station, means at said sending station for producing two sets of electrical oscillations of different frequencies, means at said sending station for transmitting said different frequencies to said receiving station, means at said sending station for alternating the transmission of said two sets of oscillations at a predetermined rate of alternation, a signal indicator at said receiving station, and a filter system associated with said signal indicator for rendering said signal indicator operatively solely in response to the arrival at said receiving station of two sets of electrical oscillations of the same frequencies as the two sets of electrical oscillations transmitted from said sending station and alternating at-the same rate as the transmission alternation.

6. In a signaling system, a sending station, a receiving station, means at said sending station for producing a plurality of paired sets of electrical oscillations of different frquencies, means at said sending station for transmitting said different frequencies to said receiving station, means at said sending station for alternating the transmission of said two sets of oscillations at a plurality of predetermined rates of oscillation, a signal indicator at said receiving station, a polarized relay for energizing said signal indicator, and means at said receiving station responsive to the plurality of predetermined rates of oscillation for operating said relay, said indicator being responsive only when the polarized relay is moved back and forth at the predetermined rate of altemation of the two sets of oscillations.

7. In a signaling system, a sending station, a receiving station, means at said sending station for producing a plurality of paired sets of electrical oscillations of different frequencies, means at said sending station for transmitting said different frequencies to said receiving station, means at said sending station for alternating the transmission of said two sets of oscillations at a plurality of predetermined rates of oscillation, a signal indicator at said receiving station, a polarized relay for energizing said signal indicator, means at said receiving station responsive to the plurality of predetermined rates of oscillation for operating said relay, and means associated with said relay for holding said relay in one of two positions corresponding to a set of electrical oscillations of one frequency until the arrival of a set of electrical oscillations of the other frequency whereupon said relay moves to the other of said positions, said indicator being of such character and so connected to the relay that it will respond only when the indicator is intermittently energized by the relay at the predetermined rate of alternation of the two sets of oscillations.

HANS P. BOSWAU. 

